Efforts have hitherto been made to use composites of fine metal particles and clay-based layered compounds in sensors.
For example, Patent Document 1 discloses technology in which a composite obtained by causing precious metal nanoparticles to aggregate within a fluid matrix such as smectite and stabilizing the aggregated state is used as a surface-enhanced Raman scattering (SERS) matrix in Raman spectroscopy that employs surface-enhancing Raman scattering. By stabilizing the aggregated state of precious metal nanoparticles within the liquid matrix, detection having a good reproducibility becomes possible.
Known methods for producing such composites includes the techniques of obtaining a composite sol (composite) of fine precious metal particles and fine plate-like particles by inducing the formation of fine precious metal particles within a dispersion of fine plate-like particles of smectite or the like dispersed in an aqueous solution or a highly polar solvent (see Patent Documents 2 and 3).
Patent Document 1: Japanese Patent Application Laid-open No. 2006-184247
Patent Document 2: Japanese Patent Application Laid-open No. H11-61209
Patent Document 3: Japanese Patent Application Laid-open No. H10-182142
However, montmorillonite group minerals (smectite, etc.), which are a leading type of clay-based layered compounds, have a layered structure created by the repetition of a three-layer structure in which a regular octahedron serves as the basic skeleton, and contain alkali metal ions as exchangeable anions between the layers. Moreover, because montmorillonite group minerals are generally hydrophilic at the surface and between the layers, they have an excellent affinity to highly polar substances such as water, dimethyl formamide and other polar solvents. On the other hand, they lack affinity with low-polarity substances such as toluene, ketone-type solvents and other solvents having a low polarity.
Therefore, although layered compound-metal particle composites having an affinity to highly polar substances can be obtained by the methods described in Patent Documents 2 and 3, obtaining layered compound-metal particle composites having an affinity to low-polarity substances is difficult.
Hence, such methods cannot be employed as film-forming processes of excellent productivity that involve dispersing a layered compound-metal particle composite in an organic solvent having an excellent volatility and low polarity to form a paste, then coating the paste onto a substrate to fabricate a desired device. Those cases in particular where a multilayer structure is produced by printing or coating the above paste present challenges in terms of work efficiency and economy.
Moreover, because layered compound-metal particle composites obtained by the methods described in Patent Documents 2 and 3 lack affinity with low-polarity substances, this becomes a problem in cases where device fabrication entails combining the layered compound-metal particle composite with an organic substance, thus limiting the application of such composites.
For example, in a case where a layered compound-metal particle composite is dispersed in the bulk heterolayer of an organic solar cell, and the photoelectric conversion ratio is enhanced by the plasmonic functionality of the metal nanoparticles, if there is no affinity between the organic semiconductor making up the bulk heterolayer and the layered compound-metal particle composite, the metal nanoparticles have difficulty approaching the bulk heterojunctions, making an adequate photoelectric conversion-improving effect impossible to obtain. Accordingly, introducing layered compound-metal particle composites obtained by the methods of Patent Documents 2 and 3 into the bulk heterolayer of an organic solar cell for the sake of improving photoelectric conversion is difficult.
For similar reasons, employing layered compound-metal particle composites obtained by the methods of Patent Documents 2 and 3 as electrode materials for electrical double-layer capacitors which use nonaqueous solvents such as ionic liquids is also difficult.